Since the first man-made satellites were flown in the late 1950s, navigation accuracy has been a fundamental performance parameter for space missions. Over the years, many techniques have been used for tracking spacecraft, with the most popular based upon measurement of the Doppler shift of the radio signals from the spacecraft. From observation of the Doppler shift profile of a spacecraft as it passes over a ground station, and from knowledge of the ground station location, the orbital elements of the spacecraft can be estimated.
The simplest radio navigation technique depends on one-way Doppler tracking using an on-board oscillator to produce a beacon that can be tracked from the ground. Early spacecraft depended on this approach. Often the beacon signal was modulated with data to provide for telemetry transmission as well. The ultimate accuracy of such one-way techniques, however, was limited by the stability of the on-board oscillator.
During the 1960s, the development of two-way coherent transponders virtually eliminated the oscillator stability problem because a highly accurate ground-based oscillator such as a hydrogen maser could be used as the frequency reference. Since their introduction, coherent transponders have been used in many spacecraft missions, with extensive ground station infrastructure built around their use. Examples include the Air Force Space-Ground Link Subsystem, the NASA Spaceflight Tracking and Data Network, and the NASA Deep Space Network (DSN).
(Note: Throughout the discussion herein, the subscripts "t" and "r" will refer to the signals transmitted and received by the ground station. The subscripts "u" and "d" will refer to the uplink and downlink signals at the spacecraft. The signals at the ground differ from those at the spacecraft due to the propagation time. All times will be referenced to a clock on the ground, even when the discussion deals with events taking place on the spacecraft.)
In a conventional two-way, coherent spacecraft navigation implementation, the ground station transmits a carrier with phase EQU .phi..sub.t (t)=2.pi.f.sub.t t (1)
where t is time and f.sub.t is the ground station transmit frequency. The transponder turnaround ratio is .zeta., so the phase of the carrier received on the ground from the spacecraft is EQU .phi..sub.r.sup.coh (t)=.zeta..phi..sub.t (t-.rho.(t))=2.pi..zeta.f.sub.t [t-.rho.(t)] (2)
where .rho.(t) is the round-trip light time for a signal received at time t. This light time will depend on the spacecraft position (relative to the ground station) at the time of the transponder turnaround. This position will not, in general, be the position of the satellite at time t due a combination of satellite travel, Earth rotation, and movement of the Earth's center through space.
The ground station generates a reference signal based on the turnaround ratio and measures a Doppler phase that is the difference of this reference and the received phase. This Doppler phase is EQU .phi..sub.Dop.sup.coh (t)=.zeta..phi..sub.t (t)-.phi..sub.r.sup.coh (t)=2.pi..zeta.f.sub.t .rho.(t) (3)
The change in the Doppler phase over some period of time is then related to the change in the round-trip light time as EQU .DELTA..phi..sub.Dop.sup.coh (t)=2.pi..zeta.f.sub.t .DELTA..rho.(t)(4)
This is the basic metric of coherent navigation. A change in the Doppler phase indicates a change in the round-trip light time, and therefore a change in the spacecraft position relative to the ground station.
While coherent tracking has proven very effective, it restricts the architecture choices for the communications hardware. Coherent transponders are typically heavy and expensive. The coherency requirement is a barrier to the renewed interest in simple transceiver-based communication systems that has been generated by the present-day emphasis on smaller, lower cost satellite missions.
For a noncoherent measurement, the phase of the signal transmitted by the spacecraft is not coherent with the uplink signal, but is a multiple, .alpha., of the spacecraft reference oscillator phase, .phi..sub.0 (t). The phase received by the ground station is EQU .phi..sub.r.sup.non (t)=.alpha..phi..sub.0 (t-.rho.(t)/2) (5)
where we have subtracted only the one-way light time from the argument of the phase reference because this phase has experienced only one-way travel.
Because we have a noncoherent system, the change in phase of the signal received on the ground will differ from what would occur with the use of a coherent system. This can result in an unacceptable error if the difference cannot be accurately removed from the measurement. Over the years, several noncoherent Doppler tracking techniques have been developed including one-way Doppler, difference Doppler, and two-way noncoherent Doppler using a subcarrier. These techniques either require highly stable oscillators (one-way Doppler), multiple ground stations (differenced Doppler), or changes to the ground station configuration (subcarrier tracking).
What is needed is a method and apparatus for two-way noncoherent navigation that provides cancellation of spacecraft oscillator drift effects and is compatible with existing ground station equipment. It must also provide accuracy equivalent to present-day two-way coherent systems (&lt;0.1 mm/second).